In recent years, increasing of the strength of iron alloy steel sheets and increasing of the use of lightweight metals such as Al alloys are being actively promoted for the purpose of reducing the weight of all manner of steel sheets for reasons such as an improvement of the fuel consumption of motor vehicles, and the like. Compared with heavy metals such as steels, lightweight metals such as Al alloys offer the advantage of a high specific strength; however, they tend to be extremely expensive, and therefore the use of such lightweight metals tends to be limited to special applications. Accordingly, in order to enable weight reduction of all manner of steel sheets to be implemented cheaply and across a broad range of steels, the strength of the steel sheets must be increased.
Because strengthening of a steel sheet is generally accompanied by a deterioration in the material properties such as the moldability (formability) and the like, an important challenge in the development of high-strength steel sheets is how to best achieve an increase in the strength without impairing the material properties. Particularly in the case of steel sheets used for motor vehicle components such as inner sheet members, structural members, underbody members, and the like, properties such as stretch flange formability, burring formability, ductility, fatigue durability, corrosion resistance, and the like are required, and how to best achieve a high degree of balance between these material properties and superior strength properties is very important.
For example, the steel sheets used in motor vehicle members such as structural members and underbody members which account for approximately 20% of the vehicle weight, are typically subjected to blanking and hole formation by shearing and punching processes, and subsequently subjected to press forming that includes mainly stretch flange formation and burring processes. Therefore, the steel sheets must satisfy an extremely stringent hole expandability (λ value) requirement.
Furthermore, in the steel sheets used for these types of members, there is a common concern that flaws or microcracks may occur on the end faces formed by the shearing or the punching processing, and that these flaws or microcracks may then develop into cracks that lead to fatigue breakdown. As a result, in order to improve the fatigue durability at the end faces of the above types of steel materials, it is necessary to ensure that flaws or microcracks do not occur.
As illustrated in FIG. 1, these flaws and microcracks that occur at the end faces tend to result in cracking in a direction parallel to the sheet thickness direction of the end face. This type of cracking is termed “peeling”. In FIG. 1, the surface of the circular cylinder represents a surface in the sheet thickness direction, and the cracking that occurs parallel to this circular cylindrical surface is termed “peeling”.
This “peeling” occurs in approximately 80% of cases for steel sheets having strength in the order of 540 MPa, and occurs in substantially 100% of cases for steel sheets having strength in the order of 780 MPa. Further, this “peeling” occurs irrespective of the hole expanding ratio (λ). For example, “peeling” occurs regardless of whether the hole expanding ratio is 50% or 100%.
Moreover, the steel sheet used for motor vehicle members such as seat rails, seatbelt buckles, wheel discs, and the like must be a high-strength steel sheet that exhibits superior esthetic appearance and superior design properties as well as excellent formability. As a result, the various steel sheets used in motor vehicle components and the like not only require the material properties described above, but may also require a stringent level of surface quality depending on the application of the steel sheet.
In order to achieve a combination of high strength and various material properties, and particularly formability, manufacturing processes have been disclosed in which, by ensuring that 90% or more of the steel microstructures are ferrite and the remainder are bainite, a steel sheet can be produced that exhibits a combination of high strength and superior ductility and hole expandability (for example, see Patent Document 1).
However, since a steel sheet manufactured using the techniques disclosed in Patent Document 1 contains 0.3% or more of Si, a tiger-striped scale pattern known as “red scale” (Si scale) tends to be generated on the surface of the steel sheet. Therefore, it is difficult to apply the steel sheet to motor vehicle components that require a strict surface quality.
Moreover, investigations by the inventors of the present invention revealed that steels having the composition disclosed in Patent Document 1 suffer from “peeling” after a punching process.
In order to address this problem, techniques have been disclosed in which, by suppressing the added amount of Si to not more than 0.3% to inhibit the occurrence of red scale, and adding Mo to reduce the size of precipitates, a high-tensile hot rolled steel sheet is obtained that has superior strength while also achieving excellent stretch flange formability (for example, see Patent Documents 2 and 3).
In the steel sheets prepared using the techniques disclosed in Patent Documents 2 and 3, although the amount of added Si is not more than approximately 0.3%, it is difficult to satisfactorily suppress the generation of red scale. And because the techniques also require the addition of 0.07% or more of Mo which is the expensive alloy element, the manufacturing costs tend to be high.
Moreover, investigations by the inventors of the present invention revealed that steels having a composition disclosed in Patent Document 2 or 3 suffer from “peeling” after a punching process.
The techniques disclosed in Patent Documents 2 and 3 make absolutely no comment relating to techniques for suppressing the occurrence of flaws or microcracks on the end faces formed by shearing or punching processing.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H06-293910    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-322540    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2002-322541